Centrifugally cast cast iron pipe



April 17, 1934. N. F. s. RUSSELL" ET AL 1,954,892

'CENTRIFUGALLY CAST CAST IRON PIPE 4 4Filed Jan. 1v, 195s g UNITED?STATES Patented pr.- 17, 1934 -PATENroFFICE CENTRJFUGALLY CAST CAST IRONml Application January-11. 1933,' semi No. 652.160

4 claims (ci. zei-'180) Our invention relates to centrifugally cast castiron pipe and has for its object to provide a pipe which will be freelymachinable and highly resistant to impact shock throughout itscylindrical portion and in which a crack, when formed, will be much lessliable to occur in straight 'lines or planes than is the-case with castiron pipes as previously manufactured, said cracks in our new pipehavinga marked tendency to occur with irregular outlines. Our new pipesare also characterized in that they have a distinctly novel andadvantageous structure. Another and practically important object whichwe 'have in view is the manufacture of pipe having pur novelconstructive features, from iron compounds or mixtures which, by reasonof their commercial availability, are usually employed in themanufacture of cast iron pipes. The essential qualities of ironcompounds, available for use in the manufacture of our pipes, are that,when cast in dry sand molds, they will form a machinable gray ironcasting and, when cast against a cold metallic surface, they will formfor some distance inward from the surface in contact with the mold, achill. Such irons can be and, for our purposes, preferably are preparedfrom such `commercially available irons or iron mixtures ofcomparatively low cost as are ordinarily used in the manufacture of castiron pipe and come within the following specification:

C. Si. S. Mn. P.

Si. S. Mn. P. T.C.

Further objects of our invention are to provide a structure having thequalities above mentioned and which will be capable of beingmanufactured 'rapidly and cheaply.

l The essential characteristic features of our new pipe consist, first,in that its cylindrical portion is made up substantially of twoconcentric annular zones, of which theouter zone extends from the outersurface ofthe pipe inward for a distance of not less than one-quarterthe thickness of the wall casting of the pipe and'consistspreponderantly of `compacted, interlacing, dendritic, crystallineformations of ferrite and/or pearlite, without unidirectional tendency,said dendritic formations being symmetrically distributed bothlongitudinally and radially of the zone, without intervening areas ofthat eutectic of cementite with austenite in various stages ofdecomposition, commonly referred to as a chill. In speaking of thedendrites as symmetrically distributed throughout the zone, we mean thatin all cross-sections of the dendritic zone the dendrites will be foundto be, in number, fineness and distribution, approximately the same,although it may also be true that the dendrites will show a tendency tobecome less fine and less numerous in the inner than in the outerportion of the dendritic zone. Another distinctive feature of the outerdendritic zone of our pipe lies in the fact that the carbon eliminatedfrom combination occurs in this zone in the form of dots or nests asdistinguished from such graphitic plates as characterize the inner zoneof the pipe and not in the form of such graphitic plates; still anotherdistinctive feature of our pipe lies in the fact that, as cast, thecombined carbon present in the outer dendritic zone is present inmarkedly smaller percentage per unit of mass than is the case as to thecombined carbon in the inner or graphitic zone of our pipe.

The graphitic structure of the inner zone of our pipe consists of amatrix of ferrite and/or pearlite, through which is distributed smallplates of graphitic carbon and in which substantially .no dendriticcrystalline structures occur, said inneror graphitic zone being furthercharacterized in that per unit of mass it contains a higher percentageof combined carbon than isthe case with the outer or dendritic zone.

It will be understood that the actual percentage of combined carbon tobe found in the cylindrical portion of pipes embodying our inventionwill varywith the composition of the metal and with the thickness of thewall casting of the pipe and these conditions will also affect therelative percentages of combined carbon per unit of mass y C. Si. S. P.Mn.

we find that the combined carbon in the dendritic zone is .16% while, inthe graphitic inner zone, the combined carbon is .44%. Again, in thecase of a pipe having a wall thickness of .64 inches, we find thatcombined carbon in the dendritic zone is .11% while, in the graphiticzone, it is .29%. We believe, from tests and experiments which we havemade, that the comparatively low percentage of combined carbon in thedendritic, as compared with the graphitic zone, of our pipes, is notonly a distinguishing feature but is a feature contributing in noinconsiderable degree, to the excellence of our pipe;

It will be noticed that in the examples which we have given showing thepercentages of combined carbon in the inner and outer zones of our pipethat the total combined carbon in the casting is less than .5% and thiswould be characteristic of most of the pipes embodying our invention andprepared to meet the ordinary requirements of the trade. In some cases,however, and to meet specific requirements for pipes to be used forcertain purposes,.it will be desirable that the total percentage ofcombined carbon should be greater and this, it will be recognized, canbe readily provided for by familiar expedients.

We have referred to the outer or dendritic structure as having a depthof not less than one-quarter the thickness of the wall of the pipe. Sucha proportionate depth of this annular zone is sufficient to impart topipesembodying our invention, and to a marked degree, the notablyincreased capacity to resist impact shocks, together with the quality ofbeing freely machinable, though we believe that with increasedproportionate depth of the dendritic zone the capacity of resistingimpact shock is materially increased and it is true that with pipeshaving thinner wall sections the proportionate depth of the dendriticzone will be greater than with pipes having thicker walls and, withcomparatively thin walled pipes, the dendritic zone will, in many cases,extend inward for approximately one-half the thickness of the pipe. Theproportionate depth of the dendritic zone will decrease with increasedwall thickness but will not be less than one-quarter of the thickness ofthe pipe with pipes having the wall thickness of centrifugally castpipe.

A fuller description of the structure of our pipe as it appears oncareful microscopic examination, is as follows:

The outer dendritic zone consists preponderantly of a continuousstructure of compacted, interlacing, dendritic, crystalline formationsof ferrite and/or pearlite without unidirectional tendency, throughwhich formations are dispersed dots and nests of carbon, asdistinguished from graphitic plates such as characterize the innergraphitic zone of the casting and small areas of iron phosphide and/oriron phosphide eutectic. The dendrites in this zone are relatively fineat or near the outer surface of the zone and have a tendency to becomesomewhat coarser and less numerous as the inner limit of the zone isapphosphide eutectic and having a structure which tends to becomecoarser toward the inner wall of the zone. Throughout this matrix, thegraphite is distributed in small dots and, more particularly, indistinct small plates of graphite.

As an aid to the understanding of our new pipe structure, we would referto the drawing forminga part of this specification, in which Figure l'is an elevation of a cast iron pipe embodying our improved structure andcomposition, with the cylindrical portion of the pipe showninlongitudinal central section and with said cylindrical portion shown aspartly broken away between the bell and spigot ends of the pipe.

Figure 2 is a cross-section through the cylindrical portion of the pipeshown in Fig. 1, taken. for example, on the section line indicated at 22 of Fig. 1.

Figure 3 is a somewhat diagrammatic showing of the structure and make-upof the outer dendritic zone of our improved pipe, as it appears in amicro-photographic enlargement of approximately 100 diameters, and

Figure 4 is, again, a somewhat diagrammatic showing of the structure andmake-up of the inner zone ofaour improved pipe, as it appears in amicro-photographic enlargement of approximately 100 diameters.

In Figs. 1 and 2. the outer and inner zones are indicated by appropriatelabels. In Fig. 3, the dendrites forming the preponderating mass of theouter dendritic zone of our pipe are, it will be understood, owing totheir interlacing and no-ndrectional disposition, sectioned in everyconceivable way in any surface prepared from microscopic examination andphotography.

In the drawing, dendritic forms lying approximately in the sectionprepared for microscopic examination, are indicated by lead linesleading to appropriate labels. While the dendrites in the dendritic zoneare preponderantly made up of ferrite, areas of pearlite will occur andthese are indicated in the drawing by lead lines leading to appropriatelabels. The occurrence in the dendritic zone of carbon dots and smallareas of phosphide eutectic are also indicated by lead lines leading toappropriate labels. As indicated in Fig. 4, the mass of the innergraphitc zone is made up of a matrixof ferrite and pearlite, as

indicated by lead lines leading to appropriate labels, through whichmatrix are distributed small plates of graphite, as indicated by leadlines and labels and in which also occur areas of phosphide eutectic, asindicated by lead lines to appropriate labels. A

Our new pipe structure can be produced successfully and economically bycasting an iron of the character described by the method invented by usand forming the subject matter of our application for Letters Patentfiled October 19, 1932, Serial Number 638,480. By this method the molteniron is progressively delivered to a rotating externally cooled steelmold through a relatively retractable runner so that the metal isdelivered to the mold in the form of a helical coil and the innersurface of the mold is progressively coated with a finely divided, drycoating material, such as ferro-silicon, in advance of the contact ofthe moltenv metal with the coated portion of the mold by means of a jetof carrier gas loaded with distributed particles of the coatingmaterial,

Causes.

which jet is relatively retracted with regard to the rotating mold so asto deposit upon the surface of the mold a helical band of the coatingmaterial in immediate advance of the contact of the molten metal withthe coated surface and, as we have pointed out in our said formerapplication, it is important that the coating of the mold should be ofuniform thickness, that the thickness, for the best results, should notmaterially exceed that minimum thickness which is found to prevent theformation of areas of chill in the casting, a thickness -which in thecase of the use of ferro-silicon as a coating material, should notmaterially exceed that formed by the delivery by the jet of carrier gasof a quantity of finely divided ferro-silicon which, if it all remainedin contact with the mold wall and was compactly deposited thereon, wouldform a coating of approximately .0003 of an inch in thickness. We havealso pointed out in our former application that the coating, forreliable results, should not exceed that formed by the uniform deliveryof the powdered material in quantity which, if it all remained incontact with the wall of the mold and was compacted thereon, would forma coating not to exceed .001 of an inch and we have also pointed out thegreat importance of laying down the coating upon the surface of the moldas short a time as possible before that surface is contacted by themolten metal and have pointed out that it is exceedingly undesirablethat the coating should be deposited upon the surface of the mold morethan six seconds before the coated surface is contacted by the moltenmetal. We have also in our former application pointed out the reasonswhich lead us to believe that the efficiency of the very thin layer ofcoating material in preventing chill and bringing about a desirablestructure in the casting is largely due to the'fact that, as the coatingis deposited, the fine particles of coating material projected againstthe surface of the mold by the carrier gas are individually coated withthin films of adsorbed gas, which films form a part of the coating as itis first formed and play an important part in the formation of thedesirable iron structure and it is, we believe, the tendency of thesegaseous films to disappear under the conditions existing in the mold,which makes it important that the coating should be laid down onthe-mold as nearly as possible immediately before the molten metalcontacts with the coated surface and accounts for the loss of efficiencyof the coating which is observable where the coating is applied by thejet to portions of the mold which are not contacted by the molten metalfor some appreciable time after the coating is applied.

We have also pointed out in our former application that a mold coatingof finely divided, dry coating material is liable, under certainconditions, to lose its continuity so as to leave areas of the moldsurface uncoated and with the result that the casting in contact withthese uncoated portions will present areas of chill. These fractures, soto speak, of the coating occur from two One, the slipping of a portionof the coating on the mold surface, due to the rotation of the coatedsurface, and the other is the tendency of the molten metal, in itsimpact against the coated surface of the mold, to push a portion of thecoating before it so as to break the continuity of the coating and bringabout areas of chill in the casting. Both of these forces tending tobring about fractures in the coating, are more injuriously effective thethicker the mold coating one-fourth the thickness of the pipe and ischar- Vquantity which, under the conditions mentioned,

would form a coating in excess of .001 of an inch in thickness.

In the formation of our improved cast pipe, it is essential thatthroughout the cylindrical porportion of the pipe there should be noareas of chill and very important that the coating should be as thin aspossible because the thickness of the coating also affects to animportant degree the depth of the dendritic zone of the casting, that isto say, the thicker the coating the less will be the depth of thedendritic zone and the greater will be the depth of the inner graphiticzone, characterized by a substantial absence of dendrites and theoccurrence of plates as distinguished from dots and unformed patches ofgraphite.

By manufacturing our pipe by the process which we have described above,it is practical, on a manufacturing scale to produce pipes havingdendritic zones of the character which we have described extendingthrough not less than onequarter of the thickness of the cylindricalwall of the pipe and this process, as applied to the casting of pipeshaving the characteristics of our new pipe, has the additional advantagethat the tensile strength of the dendritic area is high even when themetal is very hot, thus enabling the casting to resist the longitudinalstrains occurring during solidifcation and liable to result in theformation of checks in the outer surface of the pipe and, again, thenondirectional disposition of the interlacing dendrites has a markedtendency to prevent the formation of what are called pin holes in thewall of the casting, brought about by 120 the escape of occluded gases,which gases, where the crystalline structure is more or less normal tothe surface, have a tendency to follow the surface of the crystals andescape through the outer surface of the casting.

It should, perhaps, be noted that our improved pipe as cast by theprocess which we have described, is characterized by having a fairlysmooth and even outer surface which may be taken as fairly indicative ofthe symmetrical dis- 130 tributionof the dendrites in the outer zone ofthe wall of the pipe. f

Having now described our invention, what we claim as new and desire tosecure by Letters Paten is: v

1. A centrifugally cast, cast iron pipe having the compactness ofstructure characteristic of centrifugally cast pipe, the cylindricalportion of which as cast is freely machinable throughout its length andhighly resistant to impact shocks, said cylindrical portion as castbeing structurally made up of two annular zones, of which the outer zoneextends from the outer surface of the pipe to a distance of not lessthan acterized in that its iron constituent is preponderantly present insaid zone as a mass of compacted interlacing dendrites of ferrite and/orpearlite substantially without unified directional tendency, saiddendrites being symmetrically distributed both longitudinally andradially of the zone so as to form a compact and continuous structure,without intervening areas of that eutectic of iron carbide withaustenite in various stages of decomposition commonly known as a chill,said outer zone being further characterized in that the uncombinedcarbon therein is present in the form of dots and nests of carbon asdistinguished from the graphitic plates characteristic of the inner zoneand in that such graphitic plates do not occur therein, said outerdendritic zone being further characterized in that as cast the combinedcarbon therein is present in smaller percentage per unit of mass than isthe case in the inner graphitic zone. said cylindrical portion of thepipe being further characterized in that its inner, annular zoneconsists essentially of a matrix of ferrite and/or pearlite,substantially free from dendritic formations and through which matrixare distributed small plates of graphitic carbon, saidinner graphiticzone being further characterized in that as cast it contains a higherpercentage of combined carbon per unit'of mass than is the case with theouter dendritic zone.

2. A cast iron pipe as called for in claim 1, having a compositioncoming within the following specification:

C. Si. S. Mn. P.

Zim-3.85 LZ1-3.00 .05-.15 .N180 .2)-200 3. A centrifugally cast castiron pipe having the compactness of structure characteristic ofcentrifugally cast pipe, the cylindrical portion of which is freelymachlnable throughout its length and highly resistant to impact shocks,said cylindrical portion being structurally made up of twoannular'zones, of which the outer zone extends from'the outer surface ofthe pipe to a distance of not less than one-quarter the thickness 'ofvthe pipe and is characterized in that its iron constituent lspreponderantly present in said zone as a mass of compacted interlacingdendrites of ferrite and/or pearlite substantially without unitleddirectional tendency, said dendrites being symmetrically distributedboth longitudinally and radially of the zone so as to form a compact andcontinuous structure. said outer zone being further characterized inthat the uncombined carbon therein is present in the form of dots andnests of carbon as distinguished from the graphitic platescharacteristic of the inner zone and in that such graphitic plates donot occur therein, said cylindrical portion of the pipe being furthercharacterized in that its inner, annular zone consists essentially of amatrix of ferrite and/or pearlite, substantially free from dendriticformations and through which matrix are distributed small plates ofgraphitic carbon.

4. A cast iron pipe as called for in claim 3, having a compositioncoming within the following specification:

C. Si. S. Mn. P.

{iiD-3.85 1.m-3.00 .05-.15 .Z0-.80 .20-2.00

NORMAN F. S. RUSSELL. FREDERICK C. LANGENBERG.

